1. Field of the Invention
The present invention relates to a method and apparatus for creating unobtrusive indicia on image bearing media and particularly to a system employing a resistive element thermal printhead to thermally change the specularity of a marking area on the media to create indicia observable to the viewer.
2. Description of the Prior Art
The printing of high quality black and white and multiple color images on relatively glossy print media together with identification indicia, such as alphanumeric characters or spacing marks to identify the printed image, its date of printing and other information is well known in the prior art. For example, in photography, it is known to image latent frame numbers and marks on unexposed film, as taught, for example, in U.S. Pat. No. 4,553,833, which became visible after processing. In photographic printing, it is known to expose spacing marks along the sides of images printed on continuous rolls of photographic print media, to mark the reverse surfaces of photographic prints with the date of printing. It is also known to expose areas of X-ray film with latent-alphanumeric identification characters and other information applied by thermal elements energized by microcomputer stored image data as taught, for example, in U.S. Pat. No. 4,983,991, where the latent image characters become visible after processing the film. In thermal printing systems which effect a printing operation through heat transfer of dyes to a dye receiver media, it is known to print images or indicia and markings by dye transfer to the receiver media as taught, for example, by commonly assigned U.S. Pat. No. 4,710,781, and U.S. Pat. Nos. 4,995,741 and 4,516,137. It is also known from the '137 patent to employ heat sensitive media that changes color on application of heat and pressure from the thermal elements of a printhead.
In all of these contexts, particularly in relation to certain thermal printers, the printer head is connected to a variety of image generating sources. In one case, the image sources may be a series of imaging work stations that allow for generation of digital color images. These stations may be networked together so as to allow for transmission of the images from the work stations to the digital thermal printer or print engine. These images are received by the print engine and printed in accordance with a prioritizing scheme on an elongated web of thermally activated dye transfer receiver media. As a result of the prioritizing scheme, sets of interrelated images may be distributed out of sequence on the printed web media over a period of time. Alternatively, if the receiver media is in sheet form, the individual printed sheets may accumulate out of order in one or more bin.
In either case, the operator may be required to collate the related images and distribute them to various areas or customers. It is thus desirable that the printed images be individually identifiable for such distribution. It is advantageous to the operator to have the prints labeled with indicia identifying the source, content and/or distribution, as well as the date of printing. It is also advantageous for this information to be visually apparent to the trained operator but unobtrusive in other respects.
In a typical printed image identifying operation, the backsides of prints are marked using an impact printer and dye bearing web. In photographic printing, the print sequence of a set of customer negatives is memorized, and a separate stamping unit is employed downstream from the printing station to transfer dye from the dye bearing web to the appropriate location on the reverse side of the image bearing print using selectively controllable impact hammers. This method is disadvantageous when used on transparent sheets that are employed in image projection. Moreover, the printing operation may fail in the event that the dye bearing web for some reason fails.
In respect to the above-mentioned thermal printing systems, dye transfer is effected from a dye bearing donor web interposed between the thermal printhead elements and the receiver media in response to the applied thermal energy. Alternatively, thermally responsive print media is designed to change color upon application of minute amounts of thermal energy. In the former case, the separate dye transfer web suffers the drawbacks of the impact printer web, and in the latter case, the thermally responsive print media may be incompatible with the image bearing media.
In thermal dye transfer printing, the receiver media typically possesses a highly speculative or glossy finish or coating on the print surface and a dull finish or coating on the reverse surfaces of the media effected by a thin gelatin coating employed in part to decrease the effects of static electricity on the ability of the stacked sheet media to be separated and transported through the printing station. Transparency media and photographic print media are likewise usually inherently glossy on the image surface for practical or aesthetic reasons. Typical thermal photographic print media are disclosed in U.S. Pat. Nos. 4,778,782 and 4,983,991 incorporated herein by reference and are available from Eastman Kodak Company.
It is desirable therefore to provide a printing system for printing unobtrusive indicia on preexisting media that is simple and relatively failsafe.